Conventionally, as a zoom lens of a video cameras and the like, a rear-focus-type zoom lens is known that performs focusing by moving a lens group other than the first lens group on the object side. In general, a rear-focus-type zoom lens enables the first lens group to have a smaller effective diameter as compared to those zoom lenses that perform focusing by moving the first lens group, thus enabling the zoom lens to be miniaturized. Furthermore, because a rear-focus-type Zoom lens performs focusing by moving a relatively compact, light-weight lens group, the driving force to move the lens group can be small, resulting in this type of zoom lens being able to perform focusing rapidly.
As such a zoom lens, for example, there are lens systems as described in Japanese Laid Open Application H5-88083 and Japanese Laid Open Patent Application H9-281393. In these zoom lenses, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a fourth lens group with positive refractive power are sequentially arranged, in order from the object side. Zooming is accomplished by moving the second lens group, and correction for what otherwise would result in shifting of the image surface is accomplished by moving the fourth lens group.
As stated above, by adopting the rear focus method in a zoom lens, the zoom lens can be miniaturized, and also rapid focusing becomes possible. However, in a rear-focus-type zoom lens, a problem occurs in that aberration changes during zooming and focusing become large and it becomes difficult to maintain high optical performance over all object distances from nearby to infinity. To deal with this kind of problem, as a rear-focus-type zoom lens that has a favorable optical performance over the entire range of object distances, has a wide angle of view, and yet is compact, there has been disclosed a zoom lens as described in Japanese Laid Open Patent Application H10-282414, for example. However, whereas the zoom lens described therein proposes to realize miniaturization and weight reduction with a relatively small number of component lens elements, namely 11 lens elements in four lens groups, the zoom ratio is low, being only about 3.times.. Thus, a zoom lens with a higher zoom ratio is desired.